Process for the manufacture of aliphatic alpha-hydroxycarboxylic acids



United States Patent US. Cl. 260-413 4 Claims ABSTRACT OF THE DISCLOSURESaturated aliphatic a-hydroxycarboxylic acids with up to 18 carbon atomswhich are unbranched in a-position are produced by reacting thecorresponding u-nitratocarboxylic acids or their water soluble saltswith at least one mole of a water soluble salt of sulfurous acid at atemperature between 20 C. and the boiling point of the reaction mixture.

The present invention relates to a process for the manufacture ofaliphatic ot-hydroxycarboxylic acids, which are unbranched in thea-position, by reaction of the corresponding u-nitratocarboxylic acidsin an aqueous medium containing sulphites.

The a-nitratocarboxylic acids have been found to be suitable startingcompounds for the manufacture of a-hydroxycarboxylic acids, with thelatter being obtained in practically quantitative yield. Aliphatica-nitratocarboxylic acids may be obtained according to the copendingapplication Ser. No. 528,830, now Patent No. 3,324,168 in a simple andeconomical manner by reacting a-olefines with dinitrogen tetroxide inthe presence of oxygen.

In order to convert such a-nitratocarboxylic acids intou-hydroxycarboxylic acids it has previously been proposed to carry outthe saponification by treatment with dilute nitric acid or withcompounds having an alkaline reaction. This however only resulted inpoor yields since undesired oxidation reactions take place at the sametime.

In accordance with the present invention there is provided a process forthe manufacture of an aliphatic a-hydroxycarboxylic acid which isunbranched in the o position, which comprises reacting one mole of thee-nitratocarboxylic acid which corresponds to the aliphaticoz-hydroxycarboxylic acid to be manufactured and which is alsounbranched in the a-position, or a water-soluble salt thereof in onaqueous medium in the presence of at least one mole of a water-solublesalt of sulfurous acid at a temperature between 20 C. and the boilingpoint of the reaction mixture, acidifying said reaction mixture withmineral acid and separating the a-hydroxycarboxylic acid from theresulting mixture.

The crude nitrate esters, as produced on reaction of oc-olefines withdinitrogen tetroxide in the presence of oxygen, may be used directly forthe reaction, without previous purification. The nitro-compounds whichoccasionally occur as impurities in the nitrate esters do not interferewith the course of the reaction, since they react with sulfites to givesulfonic acids which are soluble in water but insoluble in ether andwhich can easily be separated off during the further working up.

In the process of the present invention any sulfite-containing compoundmay be used as long as it it water soluble. Particularly suitablecompounds are the ammonium, alkali metal and alkaline earth metalsulfites and bisulfites. Furthermore, technical ammonium sulfite lye,which may also contain a certain amount of sulfate, as

produced on purifying the waste gases from sulfuric acid manufacture,may be used as a reagent. The addition of a sulfite-containing compound,according to the invention, produces, simultaneously with the hydrolysisof the nitratocarboxylic acid to the u-hydroxycarboxylic acid, areduction of the nitrate nitrogen, whereby the sulfite is mainlyoxidised to sulfate. As a result of the reduction of the nitratenitrogen the previously mentioned undesired oxidation reactions arepractically completely suppressed. Pure a-hydroxycarboxylic acid isobtained in practically quantitative yield.

The molar ratio of sulfite to nitratocarboxylic acid preferably shouldbe at least 1:1. It is advantageous to use 2.5 moles or more than 2.5moles of sulfite per mole of nitratocarboxylic acid.

In carrying out the process of the invention, it has proved advisable tointroduce the a-nitratocarboxylic acid into the aqueous sulfitesolution. It is however also possible to add the reagents simultaneouslyto an already reacted, or still reacting, aqueous solution of a salt ofthe a-hydroxycarboxylic acid which it is intended to manufacture. Thislatter procedure is advisable when carrying out the processcontinuously. When manufacturing lactic acid it is also possible to useas the reaction medium an aqueous solution of lactic acid, to which thereagents are simultaneously added. In all variations of the process goodmixing of the reaction solution should be ensured. The amount of waterpresent in the reaction charge need not suflice to dissolve thewater-soluble sulfite, i.e. the sulfite can be used both as a solutionor with part of it in the form of a slurry. Instead of the freenitratocarboxylic acid, a water-soluble salt thereof, above all theammonium or alkali metal salt, is also suitable as a starting material.

The a-hydroxycarboxylic acid may be isolated from the reaction mixtureby any method known per se. Thus a long-chain water-insoluble acid mayafter acidification with mineral acid e.g. sulfuric acid or hydrochloricacid be directly separated from the aqueous phase. The watersolubleacids of low C-number may be obtained from the acidified reactionmixture by extraction with an organic solvent.

The a-hydroxycarboxylic acids so obtained are practically free fromimpurities and may optionally be esterified with alcohols, or acylatedwith acids, without further purification. Depending on the manufacturingconditions, the a-hydroxycarboxylic acids may contain varying amounts ofthe corresponding hemilactides, which arise by intermoleculardehydration and which may be regarded as internal esters. These are alsoa constituent of commercially sold OL-hYdI'OXYCfiI'bOXYliC acids. Thehemilactide can, by saponification with alkaline or acid materials,easily be converted to the monomolecular u-hydroxycarboxylic acid.

Straight-chain aliphatic u-hydroxycarboxylic acids, and the esters ofsuch acids produced from them, are valuable industrially usableproducts. They may for example be used as wetting agents and in plantprotection.

The following examples are intended further to illustrate the process ofthe invention.

EXAMPLE 1 447 g. of a-nitratobutyric acid (from butene-l and N 0 areintroduced over the course of 30 minutes, with stirring, into an aqueoussolution of 1512 g. of sodium sulfite in 3 litres of water which hasbeen prewarmed to 50 C., with the temperature being kept at between 70and C. by cooling. Thereafter the reaction mixture is heated to to C.and left for 2 hours at this temperature. It is then cooled to about 20C., the precipitated sodium sulfate is filtered off, the mother liquoris acidified to pH 3 with 50% sulfuric acid, and the productsubsequently continuously extracted with diethyl ether or diisopropylether. Finally, the ether is distilled off the ethereal solution, withthe a-hydroxybutyric acid remaining in a. liquid form. (Yield 297 g.=95%of theory.)

These crude products now contain maximally up to 1% water and can bedirectly further processed (e.g. esterification to butylu-hydroxybutyrate) without a purification process. B.P. 58 C./4 mm. Hg:n =1.4246.

EXAMPLE 2 135 g. of et-nitratolactic acid (from propylene and N and 1litre of technical ammonium sulfite lye (259 g./l. S0 98 g./l. NH 33g./l. S0 are simultaneously introduced, with stirring, into lactic acidwhich has been warmed to 50 C. The exothermic reaction which takes placeduring the dropwise addition is kept at 80 C. by cooling. Thereafter thereaction mixture is heated to its boiling point and kept for 2 hours atthis temperature. It is then cooled to 20 C., the precipitated ammoniumsulfate is filtered off, and the residual solution is acidified withdilute sulfuric acid and continuously extracted with ether. Thereafterthe ether is distilled from the extract, whereupon pure lactic acid(B.P. 119 C./ 12 mm. Hg) remains (yield 85 g.--95% of theory). Thislactic acid may be directly further used.

EXAMPLE 3 140 g. of crude anitratocaprylic acid (approx. 85% strength,made from octene-l and N 0 are introduced at 60 C. into a solution of270 g. of sodium sulfite in 450 ml. of water over the course of 45minutes, with stirring. The reaction is exothermic and the temperatureis adjusted to 60 C. by cooling. When addition is complete the mixtureis warmed to 90 C. to 95 C. and kept at this temperature for 3 hours.After cooling the reaction mixture is acidified with 40% sulfuric acidwhereupon it separates into 2 phases. The organic phase is washed fourtimes with 150 ml. of water and is subsequently dried over sodiumsulfate. The mother liquor and the combined wash waters are extractedwith ether in order to increase the yield, and the extract is dried andafter evaporation of the ether is combined with the organic phase. Thebulk of the impurities from the a-nitratocaprylic acid remain in theaqueous phase as sulfonic acids.

The crude product so obtained consists of free a-hydroxycaprylic acidwith varying contents of 2-(u-hydroxycaproyloxy) caprylic acid,

C H CH(OH) COOCH(C H COOH and may be directly used for most purposes,e.g. for esterification.

The yield is 95% of theory.

The pure a-hydroxycaprylic acid may be obtained by recrystallisationfrom n-heptane, and in the course of this the mother liquors which nolonger crystallise (enriched hemilactice) may, by saponification withcaustic alkali or with acid, be practically quantitatively converted tothe monomeric product.

EXAMPLE 4 345 g. of a-nitratostearic acid are introduced at 70 C. into asolution of 309 g. of sodium sulfite and 104 g. of sodium bisulfite in1400 ml. water over the course of one hour, with stirring. The reactionis exothermic and the temperature is adjusted to 70 C. by cooling. Whenaddition is complete, the mixture is warmed to 90 to 95 C. and kept atthis temperature for 7 hours. After cooling the reaction mixture isacidified with 40% sulfuric acid with stirring, whereupon the solidot-hydroxystearic acid is precipitated. The acid is separated byfiltering off,

washed with water and dried. The a-hydroxystearic acid is obtained in ayield of of theory.

The products so obtained consist of free a-hydroxystearic acid withvarying contents of 2-(ot-hydroxystearoyloxy)-stearic acid. The purea-hydroxystearic acid may be obtained by saponification of theanhydro-hydroxystearic acid with caustic alkali.

What is claimed is:

1. A process for the manufacture of a saturated unsubstituted aliphatica-hydroxycarboxylic acid with up to 18 carbon atoms which is unbranchedin the u-position, which comprises reacting one mole of a compoundselected from the group consisting of the aliphatic a-nitratocarboxylicacid, which corresponds to the aliphatic a-hydroxycarboxylic acid to bemanufactured and which is also unbranched in the a-position, and thewater soluble salts thereof in an aqueous medium in the presence of atleast one mole of a water soluble salt of sulfurous acid selected fromthe group consisting of ammonium, alkali metal and alkaline earth metalsulfites and bisulfites at a temperature between 20 C. and the boilingpoint of the reaction mixture, acidifying said reaction mixture withmineral acid and separating the u-hydroxycarboxylic acid from theresulting mixture.

2. A process for the manufacture of a saturated unsubstituted straightchain aliphatic a-hydroxycarboxylic acid with up to 18 carbon atoms,which comprises reacting one mole of a compound selected from the groupconsisting of the aliphatic u-nitratocarboxylic acid, which correspondsto the aliphatic a-hydroxycarboxylic acid to be manufactured and thewater soluble salts thereof in an aqueous medium in the presence of atleast 2.5 moles of a water soluble salt of sulfurous acid selected fromthe group consisting of ammonium, alkali metal and alkaline earth metalsulfites and bisulfites at a temperature between 20 C. and the boilingpoint of the reaction mixture, acidifying said reaction mixture withmineral acid and separating the a-hydroxycarboxylic acid from theresulting mixture.

3. A process according to claim 2, in which any content of hemilactideof the hydroxycarboxylic acid present in the product of reaction isconverted into the free monomeric acid by saponification with causticalkali.

4. A process according to claim 2, in which the a-nitratocarboxylic acidis introduced into an aqueous solution of the sulfite.

References Cited UNITED STATES PATENTS 2,847,453 8/1958 Gardner et al.260-466 2,847,465 8/ 1958 Robertson et al. 260-533 3,324,168 6/1967Miiller et al. 260-467 FOREIGN PATENTS 1,189,972 4/ 1965 Germany.

855,880 12/1960 Great Britain.

OTHER REFERENCES Jones, Principles of Organic Chemistry, 1906 p. 231,The MacMillan Company.

Jones, Inorganic Chemistry, 1947, p. 403, The Blakiston Company.

Morrison et al., Organic Chemistry, 1959, p. 438, Allyn & Bacon. ALEXMAZEL, Primary Examiner.

R. V. RUSH, Assistant Examiner.

U.S. Cl. X.R. 260-533, 535

